CAMBRIDGE — It is a powerful mural, featuring black and white Americans sleeping on floors, marching, and being beaten by police during Depression-era protests. But for eight decades, Lewis Rubenstein and Rico Lebrun’s sprawling fresco has remained out of public view.

That changes this November, when the Harvard Art Museums reopen after a six-year, $350 million renovation and expansion. Designed by renowned architect Renzo Piano, the project brings all three of the Harvard Art Museums — the Fogg Museum, Busch-Reisinger Museum, and Arthur M. Sackler Museum — under the same roof for the first time. It has also transformed the complex, increasing gallery space by 40 percent and adding study centers, a cafe, and a dramatic, pyramid-like glass roof.

The reinstallation of the 10-by-5-foot mural marks a key milestone. But it’s just the start. Thousands of artworks, in storage off site during the construction, are heading back to Quincy Street.

“The site has been closed for a long time now, and so having the building to the point it’s being handed back to us is thrilling,” said Deborah Martin Kao, the Harvard Art Museums’ chief curator. “It’s a huge boost.”

These days, the Harvard Art Museums look like a brand new house before move-in day. The 240,000-square-foot building is well-lighted, the floors freshly finished. Stroll through the new, glass-walled study centers and past the cabinets in the Conservation Department; they are clear and empty. Downstairs, in the museum’s central courtyard, workers unpack and assemble glass cases built in Italy.

The Harvard Art Museums have an acclaimed collection of 250,000 objects, ranging from ancient artifacts from the Middle East to contemporary works by Yoko Ono and Christo. About 2,000 pieces will be on view, including some that Harvard has never publicly displayed before.

“It’s funny, it’s kind of an odd interval,” said Tom Lentz, Harvard Art Museums director. “We’re still refining parts of the building. But at the same time, we can actually get a sense of the promise and potential of the building. It’s an exciting time and a kind of nerve-wracking time. We’ve announced our public dates, so we have to hit our deadline.”

The daily progress now is not as dramatic as during construction, when bulldozers and bucket trucks rumbled over a muddy pit. These days, advances are measured in the details. Last week, signs were installed marking the entrances. While offices of the curators remain in a satellite building in Somerville, they visit the galleries regularly to oversee the installation.

“Until all of the art is installed, it’s still a kind of empty vessel,” said Lentz. “A beautiful, empty vessel but not complete.”

The “Hunger March” fresco is the first artwork to be installed. On a recent morning, three conservators stood on a scaffold, surrounded by paint trays, cleaning chemicals, and cotton swabs. They worked to remove dirt and “fixes” made by workers in the past. The idea was to reveal what the artists had created.

But this was no simple restoration. The “Hunger March” is a true fresco, meaning that it was painted into wet plaster, a technique popularized during the Italian Renaissance. Think of Michelangelo’s work in the Sistine Chapel. True frescos are quite rare in this region. There are none at the Museum of Fine Arts or the Isabella Stewart Gardner Museum. Even John Singer Sargent’s famous “Triumph of Religion” cycle at the Boston Public Library was painted on panels in England and then sailed to the United States.

Because the “Hunger March” mural was painted into an 80-year-old wall on the fourth floor, moving it out of the building to storage — and then back, to a first-floor gallery space — was a true feat of engineering. In fact, conservators were initially against the move. But ultimately they had no choice. All the construction work put the piece at risk.

“You don’t go on Google and say, ‘How do I move a fresco?’ ” said Claude LeBlanc, general superintendent at Skanska USA, the project’s construction manager.

Curators and conservators worked with Skanska USA on a plan to slice a section of the existing wall, transport that 5,000-pound slab on a flatbed truck to a storage facility, and then, when the renovation and expansion were largely done, bring it back to Cambridge.

“We were mostly concerned with protecting the surface and preserving and cleaning it and consolidating it,” said Hensick. “We had to find a material that could protect it during this move.”

The Conservation Department settled on cyclododecane, a chemical that has only been used in recent years for conservation. It is a crystal-like substance that becomes a clear liquid when heated and, after being brushed onto the surface of a painting, will turn into a waxy solid. This offers protection to the work while it is being moved or stored.

“It’s like something you might look for in a detective store,” said Hensick.

Rubenstein, a Harvard graduate, painted the mural in 1933 with Lebrun, an Italian-born artist and his mentor. The previous winter, he had gone on a hunger march, one of countless protests held by men who were out of work. There Rubenstein encountered “black and white men together,” he wrote, sleeping on church floors and sidewalks, and facing beatings at the hands of the police. Those are the images he re-created on the museum wall.

The mural was originally installed in a fourth-floor painting studio, a space that was later converted to a conservation lab. So it has never been seen by the general public.

“It’s unusual and incredibly important to have in Boston,” said Kao. “From a technical perspective, it’s quite fascinating. From an art historical perspective, it was painted during the depths of the Depression and is dedicated at looking at the working masses, not the aristocratic elite.”

In some ways, the complexity of the “Hunger March” mural installation mirrors the complexity of the renovation and expansion as a whole. As part of the project, the 86-year-old facade of the Fogg Museum on Quincy Street needed to be maintained. But an earlier version of the Busch-Reisinger Museum, added in 1991 behind the Fogg, facing Prescott Street, had structural problems and was knocked down. In addition, Harvard decided to move the Sackler Museum, which was in a building across Broadway, into the expanded complex.

A glass seam now visually separates the wood exterior of the new construction from the brick of the old, but inside the museums flow seamlessly through a central staircase and the renovated Calderwood Courtyard. The courtyard has been extended upward, with glass arcades on the top three ﬂoors, and a glass roof that the architect calls a “Light Machine.”

Museumgoers can enter from Quincy Street or Prescott Street to the ground ﬂoor, which includes a new shop and cafe that will be open to the public without a museum ticket. There is also a 300-seat lecture hall, and, upstairs, new art study centers that will offer visitors access to tens of thousands of works of art by appointment.

With the extra gallery space, a range of artworks that have never been on view at Harvard or have not been seen in a long time will be on display, ranging from centuries-old pieces from India and China to works by Copley, Miro, and Diane Arbus.

One day last week, Lentz hustled to a meeting in the complex’s Naumburg Room, known for its wood panels, dramatic balcony, chandelier, and several stained-glass windows. This period room, donated to the Fogg, has been moved across the complex and rebuilt. The stained glass is being cleaned and conserved by a specialist in New York, and curators recently selected the hand-blown glass that will surround the historic panels.

Meanwhile, Lentz said, discussions continue about the placement of the Romanesque capitals in the renovated Calderwood Courtyard. They are currently in storage.

But so much has been settled. The staff of all three art museums will be together for the first time. The Fogg, cooled for years by floor fans, is now properly climate-controlled. And the museum’s courtyard can now function as a meeting space, where visitors from far and wide can gather at cafe tables.

On a recent afternoon, Michael Morgan, a plasterer from T.J. McCartney, Inc., kneeled next to a white wall, smooth except for a few pieces of blue painter’s tape meant to mark imperfections. He slathered joint compound onto those spots.

“I’ve been here for a while, probably seven months now,” he said. “You can see the end.”

BostonGlobe.com complimentary digital access has been provided to you, without a subscription, for free starting today and ending in 14 days. After the free trial period, your free BostonGlobe.com digital access will stop immediately unless you sign up for BostonGlobe.com digital subscription. Current print and digital subscribers are not eligible for the free trial.

Thanks & Welcome to Globe.com

You now have unlimited access for the next two weeks.

BostonGlobe.com complimentary digital access has been provided to you, without a subscription, for free starting today and ending in 14 days. After the free trial period, your free BostonGlobe.com digital access will stop immediately unless you sign up for BostonGlobe.com digital subscription. Current print and digital subscribers are not eligible for the free trial.